Spong, U.S. Ser. No. 668,495, filed Mar. 19, 1976, now U.S. Pat. No. 4,097,895 has described a recording medium which comprises a light reflecting material, such as aluminum or gold, coated with a light absorbing layer, such as fluorescein which is operative with an argon laser light source. The thickness of the light absorbing layer is chosen so that the structure has minimum reflectivity.
An incident light beam ablates, vaporizes or melts the light absorbing layer, leaving a hole and exposing the light reflecting layer. After recording, at the wavelength of the recording light there will be maximum contrast between the minimum reflectance of the light absorbing layer and the reflectance of the light reflecting layer. Further, when the light reflecting material is itself a thin layer on a non-conductive substrate, little energy is lost either by reflection from the thin absorbing layer or by transmission through the reflecting layer. Thus, the energy absorbed from the light beam is concentrated into a very thin film and recording sensitivity is surprisingly high.
Because of its low input power requirement, small size and its capability for direct modulation of its optical output power by modulation of the electrical drive current, a solid state injection laser, in particular the AlGaAs laser which operates in the wavelength range from about 750 to 850 nanometers (nm), is a preferred light source for an optical recording system. Thus, materials which ablate, vaporize or melt at low temperatures upon absorption of optical energy in this wavelength range would be most useful in an optical recording system.
In order to be useful as a light absorbing layer for the above-described recording medium, materials must be able to be applied to a substrate in the form of a thin, smooth layer of high optical quality and a predetermined thickness; they must be absorbing at the frequency of the optical source employed; and they must ablate, vaporize or melt to form uniform holes.
Bloom et al, U.S. Ser. No. 837,853, filed Sept. 29, 1977, have described an ablative optical recording medium operative with an AlGaAs laser which comprises a substrate coated with a light reflecting layer, which in turn is coated with a light absorbing layer selected from the group consisting of lead phthalocyanine, chloroaluminum phthalocyanine, vanadyl phthalocyanine, stannic phthalocyanine and chloroaluminum chlorophthalocyanine.
These materials have the disadvantage, however, that their ablation temperature is in the range from 300.degree. C. to 400.degree. C. Materials with a lower ablation temperature would require less energy to reach this temperature and would thus be more sensitive. Thus, materials which absorb light between 750 nm and 850 nm, form specular, amorphous films and have a low melting temperature would be a significant improvement in the art.